In situ Fracture and Adhesion Failure of Al-Cu-Fe Quasicrystalline Films
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IN SITU FRACTURE AND ADHESION FAILURE OF Al-Cu-Fe QUASICRYSTALLINE FILMS M. J. Daniels1, B. L. French1, David King2, J. C. Bilello1 1 Center for Nanomaterials Science, Department of Materials Science and Engineering, University of Michigan, 2300 Hayward St., Ann Arbor, MI 48109-2136 2 Technology Assessment and Transfer, Annapolis, MD, 21108 ABSTRACT Quasicrystalline precursor coatings were deposited on single crystal silicon and sapphire wafers by RF sputtering from an AlCuFe powder composite target. Synchrotron white beam radiography/topography and stress analysis were performed in situ on the wafers during heating to 495 or 585°C, and subsequent cooling. A plateau region of constant stress was present throughout most of the 1 hour anneals before a large tensile stress developed in the film during cooling due to coefficient of thermal expansion mismatch. Cracking was observed for films on both substrates at an average film stress of approximately 930 MPa. Distinct differences in the fracture behavior were observed for the two different substrates. X-ray diffraction performed on films after annealing suggested that texturing took place during the transition to a fully developed quasicrystalline structure. INTRODUCTION Quasicrystals have been investigated for a variety of applications including low friction coatings due to their low measured coefficients of friction and hardness of the quasicrystalline structure [1]. This hardness arises out of an inability to pass dislocations through the aperiodic quasicrystalline lattice [2]. Coatings suitable for tribological applications are generally produced on the micron scale, however, the methodology of producing quasicrystalline coatings is relatively new. Flame spray methods have been used to deposit a thick quasicrystalline coating by concurrently melting quasicrystalline particles and propelling them onto a substrate [3]. Sputtering from a magnetron target has also been used to deposit a precursor film which was subsequently annealed above 500°C to form a large-grained quasicrystalline or approximant structure [4]. Approximant phases are crystal structures that closely resemble their quasicrystalline counterparts, but are periodic on large length scales. In the AlCuFe system, the icosohedral phase is produced above 600°C, while a rhombohedral approximant (r-phase) is produced between 450°C and 600°C [5]. Due to the similarity in structure, the r-phase is believed to have properties similar to those of the quasicrystal. Regardless of the method used to deposit quasicrystalline coatings, little is known about their mechanical behavior. In the current work, transmission Laue diffraction radiography/topography was performed on AlCuFe magnetron sputtered coatings on silicon and sapphire substrates in order to examine the response on heating and cooling. An in situ method for determining the average film stress during thermal cycling was used to show the stress state concurrently with the observed behavior.
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EXPERIMENT Coatings were produced by RF sputtering
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